1. Field of the Invention
The present invention relates to a self-biasing magnetoresistive spin valve sensor that does not require an antiferromagnetic layer for pinning the magnetization of a pinned layer, but, in contrast, employs fields from the sense current to pin the magnetization of the pinned layer and unbias the magnetization of the free layer.
2. Description of the Related Art
A spin valve sensor is a type of magnetoresistive (MR) sensor that is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a magnetic disk or a magnetic tape. A spin valve sensor includes a non-magnetic conductive layer, hereinafter referred to as a spacer layer, that is sandwiched between first and second ferromagnetic layers. The first ferromagnetic layer is also referred to as a "pinned layer" and the second ferromagnetic layer is also called a "free layer". First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the first ferromagnetic layer is pinned 90.degree. to the magnetization of the second ferromagnetic layer, and the magnetization of the second ferromagnetic layer is free to respond to external magnetic fields. The thickness of the spacer layer is chosen to be less than the mean free path of conduction of electrons through the sensor. With this arrangement, some of the conduction electrons are scattered by the interfaces that the spacer layer shares with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal. Scattering is highest when the magnetizations of the pinned and free layers are antiparallel. The amount of scattering changes the resistance of the spin valve sensor proportional to cos .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. A spin valve sensor has a significantly higher magnetoresistive (MR) coefficient than an anisotropic magnetoresistive (AMR) sensor. For this reason it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
A spin valve read head is typically combined with an inductive write head to form a combined head. The combined head may have the structure of either a merged head, or a piggyback head. In a merged head the second shield serves as a shield for the read head and as a first pole piece for the write head. A piggyback head has a separate layer which serves as the first pole piece for the write head. In a magnetic disk drive an air bearing surface (ABS) of the combined head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode, the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When the sense current is conducted through the spin valve sensor the resistance changes cause potential changes that are detected and processed as playback signals.
The magnetization of the first ferromagnetic layer is typically pinned by exchange coupling with an antiferromagnetic layer. The antiferromagnetic layer may be constructed from a group of materials which include FeMn, NiMn and NiO. The blocking temperatures of these materials falls in a range from 160.degree. to 200.degree. . Blocking temperature is the temperature at which the magnetic spins within a material lose their orientation. When the blocking temperature of the antiferromagnetic material is exceeded the spins of the antiferromagnetic layer lose their orientation causing the first ferromagnetic layer to no longer be pinned. Unfortunately, the aforementioned blocking temperatures can be easily exceeded by electrostatic discharge (ESD) or electrostatic overstress (EOS) during fabrication, testing or operation in a disk drive. ESD can ruin the spin valve sensor, while EOS can reduce its efficiency. Another problem with the prior art antiferromagnetic layer is that it shunts a portion of the sense current thereby reducing the efficiency of the sensor.